Sponges are definitely a good sign, and yes, pineapple sponges are very funky looking I had a BUNCH on the underside of a rock once, and was glad they never spread anywhere visible in the tank. If you ever want to get rid of them, you can always pluck or scrape them off the rock, but they are excellent filters. You can always remove them and put them in your sump or fuge (without exposing them to air, which will most likely kill them).